This invention relates to a hydraulic system for raising and lowering an aircraft landing gear, and more particularly but not exclusively useful for a kind of aircraft landing gear known as a side brace, where the landing gear is movable between a generally vertical condition for landing, and a generally horizontal condition for stowage e.g. within a wing of the aircraft.
A particular feature of such side brace landing gears is that landing loads act through an actuator which is provided to raise and lower the landing gear. Accordingly such actuators have to be more substantial than would be required simply for raising and lowering the landing gear. Typically such actuators are piston and cylinder arrangements, and the piston diameter is made larger than is necessary just for raising and lowering the landing gear.
As a result, th actuator demands a large volume of hydraulic fluid for operation. Particularly during landing, when it is desired to lower the landing gear, other aircraft services will be demanding hydraulic fluid, for example flap lowering actuators may require fluid, which can place high demands on the hydraulic pump(s) supplying pressurised hydraulic fluid to the various services.
According to one aspect of the invention we provide a hydraulic system for raising and lowering aircraft landing gear, the system including an actuator which is extendible and retractable to operate the landing gear, the actuator including a movable member in a casing, the movable member being moved relative to the casing in a first direction to extend the actuator when fluid under pressure is supplied to a first side of the movable member while fluid is exhausted from a second side of the movable member, and the movable member being moved in a second direction to retract the actuator when fluid under pressure is supplied to the second side of the movable member while fluid is exhausted from the first side of the movable member, and there being selector valve means selectively to supply pressurized fluid to the first or second side of the movable member, and means provided to permit exhausted fluid from at least one of the first and second sides of the movable member to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable member.
By virtue of the system of the present invention, the demand on the hydraulic pump(s) is thus reduced during landing gear operation as exhausted fluid is recirculated to augment the fluid supply, thus reducing the volume of fluid required from the hydraulic pump(s) to operate the landing gear.
In one arrangement the means which permit exhausted fluid to augment the supplied fluid includes a check valve which is opened as tile movable member of the actuator moves relatively in the casing in the first direction to extend the actuator and lower the landing gear.
The check valve mast be adapted to open to permit exhausted fluid from the second side of the movable member to augment the supplied fluid in response to the pressure of the fluid supplied to the first side of the member or alternatively in response to a pressure build up in a line carrying exhausted fluid from the second side of the movable member. In each case preferably closure means are provided positively to close the check valve when pressurised fluid is supplied by the selector salve to the second side of the movable member.
Such closure means may be of a hydraulic and/or mechanical nature.
Preferably means are provided to relieve exhausted fluid which is not recirculated from the at least one of the first and second sides of the movable member as the movable member reaches tile end of travel in the casing. Thus there is no risk of trapped fluid interfering Faith the propel operation of the landing gear.
In one arrangement the hydraulic system includes a first fluid supply line to the first side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a first position, and a second supply line to the second side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a second position, and the means which permit exhausted fluid from at least one of the first and second sides of the movable member to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable member, permitting the exhausted fluid to flow from the second supply line to the first supply line.
To ensure that exhausted fluid is available to augment the supplied fluid to extend the actuator, the second supply line may include non-return means at least to restrict the flow of exhausted fluid from the hydraulic system. However desirably a restrictor means is provided to enable a restricted flow of exhausted fluid to by-pass the non-return means so that fluid which is not recirculated, is not trapped in the second supply line which could interfere with the proper operation of the landing gear.
The selector valve means may be movable to a first position to permit the flow of fluid therethrough from a source of pressurized fluid to the first side of the movable member, and to a second position to permit the flow of fluid therethrough from the source to the second side of the movable member, and to a rest position in which the source is isolated and fluid may pass from the system tank.
According to a second aspect of the invention we provide an aircraft having landing gear which is raised and lowered by a hydraulic system according to the first aspect of the invention.
According to a third aspect of the invention we provide a valve including, a valve member and a piston each received in a passage in a valve body, the valve member and piston being biased apart by resilient means such that the valve member is urged towards a valve seat towards one end of the passage, and the piston is urged towards a stop towards an opposite end of the passage, a fluid inlet and a fluid outlet, the pressure of fluid at the inlet when sufficient, acting to move tile valve member against the force of the biasing means off the valve seat to permit fluid flow from the inlet, past the valve seat, to the outlet, and the piston being movable in the passage away from the stop in response to a pilot pressure delivered to a pilot pressure port of the body against the force of the biasing means to a position in die passage in which the piston engages the valve member and restrains the valve member against movement off the valve seat in response to the inlet pressure.
Means may be provided to permit fluid pressure at the outlet to be communicated to an intermediate region of the passage between the valve member and the piston at least when the piston is engaged with the stop. For example the outlet may open into the valve passage and there may be a flow path for the fluid at the outlet past and/or through the valve member to the intermediate region when the valve member is in engagement with the valve seat.